Electrical Transformers 101: A Guide To The Fundamentals


In the simplest of terms, power transformers make the generation, transmission, distribution and consumption of AC electric power possible within a multi-voltage power system. They are critical components in power systems that are used to transfer electric energy between two or more circuits through electromagnetic induction, and this is exactly the primary purpose of the power transformers.

Why are power transformers so important in the electricity transmission? The power is generated and transmitted at high voltages, then it need to be stepped down to lower distribution voltages before reaching end users. Because high voltages are needed for transmission over long distances, since the power lost through transmission lines is proportional to the square of the current (I2), higher transmission voltages greatly reduce line losses. This allows more of the generated power to reach loads while lower voltages are required for safe consumption by residential and commercial loads. Transformers are used to convert between voltage levels as needed. Transformers allow utilities to transport power over long distances with minimal line losses, then reduce voltages for safe consumption.



There are many essential parts and configurations of the power transformers collectively supporting the machine working successfully. Normally if we despite different design goals, they could be divided into:

  • Core: The core provides a path for the magnetic flux. It is made up of stacked steel sheets to reduce energy loss from eddy currents. The core shape can be square, rectangular or round.
  • Windings: Wire coils are used to transform voltages. The primary winding gets the input voltage while the secondary winding outputs the transformed voltage. They are wrapped around the core. Windings play an essential role in the working process of the transformers: when AC power enters the transformer, it first flows through the primary winding.
  • Insulation: Special insulation that can withstand high voltages is used to insulate the windings and cores. Common types include oil-impregnated paper and high-density pressboard.
  • Cooling system: Transformers generate heat during operation, so a cooling system like oil, air or water is used to release the heat and prevent overheating. Cooling helps extend the transformer's lifespan.
  • Tank: The tank contains all the transformer components and the cooling medium. It is usually made of steel or aluminum, properly sealed and able to withstand high pressures.
  • Bushings: Bushings are installed at the top of the tank to allow external power lines and wiring to connect with the transformer windings. They safely insulate and seal these connections.
  • Tap changer: A tap changer controls the output voltage. By changing the tapings on the transformer, the voltage ratio can be regulated to provide the required output.

Power transformers are key elements in power grids for voltage conversion and transmitting power over long distances with little loss. They come in a range of sizes and power ratings depending on the use. Transformers also need frequent monitoring and maintenance to minimize failure risk and ensure maximum efficiency.



Power transformers work based on the principles of electromagnetic induction. 

The basic steps of how power transformers work are:

  1. AC power enters the transformer, flowing through the primary winding. The primary winding comprises coils of wire around an iron core.
  2. The alternating current in the primary winding produces a constantly changing magnetic field in the core. The strength of the magnetic field depends on the voltage and number of turns of wire in the primary winding.
  3. The magnetic field from the primary winding cuts across a secondary winding, also made of coiled wire wrapped around the core. This induces an alternating voltage in the secondary winding.
  4. The strength of the induced voltage in the secondary depends on the number of turns in its winding. More turns result in higher voltage, while fewer turns yield lower voltage. This allows transformers to convert to either higher (step-up) or lower (step-down) voltages.
  5. The current then flows out of the transformer at the converted voltage level, either higher for power transmission or lower for power distribution.
  6. Transformers use alternating current because the magnetic field constantly changes direction, inducing voltage reversal in the windings and allowing power to be transformed. With direct current the magnetic field is static so no voltage would be induced.

The key factors that determine how much a transformer can step-up or step-down voltage are the turns ratio between primary and secondary windings and the core's magnetic permeability. Transformers use iron cores because iron's high permeability means more of the magnetic field can cut across the secondary winding. 


The working principle of any transformer is based on converting electrical energy to magnetic energy, transferring that energy between isolated circuits, then converting it back to electric energy - all through the interactions between the primary winding, core and secondary winding. The design and configuration of these components ultimately determine the specific operation and function of the transformer. 



There are many variations and applications of power transformers, but these represent the major broad types according to their general shape, cooling medium, function and end uses. Power transformers come in a variety of types for different applications, including step-up transformers, step-down transformers, distribution transformers, power transformers, substation transformers, rectifier transformers, and instrument transformers.


  1. Step-up transformers - Increase the voltage while decreasing the current. Used at power generation stations to raise the voltage for transmission on high voltage power lines.
  2. Step-down transformers - Decrease the voltage while increasing the current. Used to reduce transmission voltages to distribution voltages, and even lower for direct use by consumers. Most common type of transformer.
  1. Distribution transformers - Specifically designed for reducing voltages used on distribution lines to levels suitable for commercial and residential use. Typically mounted on poles or on/in ground.
  1. Power transformers - Handle the bulk transfer of power in transmission networks. Very high capacity, often oil-cooled. Connect lines operating at different voltage classes.
  1. Pad-mounted transformers - Ground-mounted step-down transformers located close to the loads they serve. Weather and tamper-resistant for residential neighborhoods or industrial areas.
  1. Substation transformers - Very large step-down transformers used at electrical substations to convert transmission voltages to subtransmission or distribution voltages (e.g. 115 kV to 35 kV). Often liquid-filled.
  1. Rectifier transformers - Step-down transformers used to provide an AC input to rectifiers for DC power conversion. Require highly-insulated windings that can withstand harmonics.
  1. Generator step-up (GSU) transformers - Step-up transformers used to directly connect AC power generators to high-voltage transmission networks. Require electrostatic shielding to prevent interference. Often air-cooled.
  1. Instrument transformers - Step-down transformers specifically used to power measuring instruments, meters and relays in power systems. Provide current and/or voltage reduction to levels suitable for power monitoring equipment.
  1. Grounding transformers - Provide a neutral ground reference and current path for limiting voltages due to lightning strikes or other imbalances in delta (3-phase) power systems. Ensure safety of equipment and personnel. 
  1. Autotransformers - Special type of power transformer with common single winding. Use part of the winding at the input voltage and part at the output voltage. More efficient than isolating transformer for small voltage changes. Used for power factor correction or in industry.



In summary, power transformers range in size from small single-phase pole-mount units to very large three-phase substation transformers in electrical grids. Oil, air or water are commonly used as cooling media depending on transformer design and environment, they convert, isolate and match power to get it where it needs to go. Though unseen, they are the heart of any power grid and fundamental to electrical power utilization across the world. Our modern society owes its existence to these transformer technologies that make energy from generation to consumption possible. 

Modern society has come to utterly depend on the benefits of electricity, but power cannot be generated and delivered over a vast transmission network without the crucial ability to transform its properties as needed through various voltage levels. Today, power transformers operate behind the scenes to provide this ability and enable our modern way of living with electricity - powering homes, businesses, communication systems, transportation networks and more, day and night. They are truly transformer heroes of the electrical world.


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